Blowing machine

[Terry McGee]

At the risk of sounding foolish . . . wouldn't it be nice to have a single flowmeter we could trust ?

It would certainly be a lot easier to have a singe flowmeter that covered the range and had the resolution we need. In the same way that reading the Digital Manometer is so much easier than trying to judge which part of the meniscus to believe. But, consider this. We are now aware that if we perch the floating bead flow meter at a pressure higher than atmospheric, we introduce a source of error due to the denser air. The digital flow meter you were looking at is based (if I understand correctly) on cooling effect of air flow over a heated sensor. What happens to that cooling effect if we increase the pressure and the density of the air flowing through it? Do they talk about that in the specs?

I imagine that at least its resistance to air flow is probably pretty low. Again, do they talk about that in the specs?

I have been wondering if we should concentrate on measuring pressure rather than flow for our tests. I know Tunborough prefers flow, but if we could use calibrators to predict the relationship, the Manometer is non-intrusive and easy to use. But lets see where the journey takes us....

[david_h]

Thanks Tunborough for the confirmation and revised model.
This part

For the "resistance", resistance = sqrt(P)/Q = 0.2657 * h * w / sqrt(rho * K) = 0.2657 * h * w * Cd / sqrt(rho)

allows something I has been working towards saying but avoiding because of the complication of explaining where a number 4.1029 came from. If we rearrange that and set K=1 for a zero length windway and ignoring the inlet air speed as in the previous model we get

h*w= Resistance *sqrt(rho)/0.2657 = Resistance*4.1029

That's the area of a hole in a thin sheet of metal or plastic that we can blow through to get a feel of what a whistle with a quoted Resistance feels like. The 4.1029 is a messy function of the density of air, the density of water, the number of seconds in a minute and various metric unit conversions all of which appear in Tunborough's model.

We can take a PET soft drink bottle, drill an appropriate diameter hole in the cap, glue the cap back on and cut the bottle to make something like a funnel (I'll explain all that later). A full forced inhale-forced exhale breath blown through it in 15 seconds is in the ball park of 12 L/min, depending on one's lung capacity.

So when C&F has a sticky with measured McGee Resistances for various whistles we can get a feel for what they are like in terms of a hole in a bottle cap. We still won't know how hard we have to blow for a particular note but we when we do we could work out roughly how long we could hold, say, a low G for.

The cone of bottle is a health and safety issue as the temptation to suck may be strong and breathing though a 3.94mm orifice in the windpipe until the medics remove the bottle cap might involve a sense of fear and breath control practice we could do without.

[david_h]

I have been wondering if we should concentrate on measuring pressure rather than flow for our tests. I know Tunborough prefers flow, but if we could use calibrators to predict the relationship, the Manometer is non-intrusive and easy to use. But lets see where the journey takes us....

I haven't absorbed the last tranche of posts yet but I think if the focus was on comparing existing whistles, or practical tinkering with the engineering, then measuring just pressure might serve. 'stringbed' has pointed out that it serves the recorder makers well. I'm hoping a zero length calibrator with remove enough potential contributions to K that we get an good idea which oddities are coming from the flow meters. Looking at all the fluid dynamics complications Tunborough is turning up what is going on in those flow meters, especially as the space above the ball gets smaller, is probably not simple. Or maybe their maker has trouble with its reamers.

[Tunborough]

I have been wondering if we should concentrate on measuring pressure rather than flow for our tests. I know Tunborough prefers flow, but if we could use calibrators to predict the relationship, the Manometer is non-intrusive and easy to use. But lets see where the journey takes us....

I haven't absorbed the last tranche of posts yet but I think if the focus was on comparing existing whistles, or practical tinkering with the engineering, then measuring just pressure might serve. 'stringbed' has pointed out that it serves the recorder makers well. I'm hoping a zero length calibrator with remove enough potential contributions to K that we get an good idea which oddities are coming from the flow meters. Looking at all the fluid dynamics complications Tunborough is turning up what is going on in those flow meters, especially as the space above the ball gets smaller, is probably not simple. Or maybe their maker has trouble with its reamers.

With the anomalies we are seeing with the flowmeters in different configurations, I'm prepared to abandon them. We may not need the fudge factor that I've called Ke to get a good measure of air speed with the manometer alone. Before they go, though, I would appreciate trying a few more of the calibrators upstream of the flowmeter instead of downstream. 5, 10, 15, and 20 L/min should be enough to put Ke to rest.

[Terry McGee]

So running the Manometer in Differential Mode. I still worry that the elevated pressure due to the high back pressure of the Flow Gauge will impact on the Calibrator results. (Convince me I'm wrong!) Indeed, have we ever measured that back pressure? I suspect not. Give me a few minutes....

Woah, interesting. So, Pressure Regulator via Whistle Connector (to take advantage of its Pressure Takeoff to the Manometer) through a Flow Gauge to air. I can only get to around 14-15L/Min when I run out of Manometer (1400mm H20). Tried both flow gauges, just in case.

What would you think of this setup and operating procedure for your Calibrator tests? Pressure Regulator via Flow Gauge to Whistle Connector (and Pressure Takeoff to the Manometer) through Calibrator-under-test to T-junction with Pressure Takeoff point to Vacuum pump. Manometer in Differential Mode, but at first connected only to upper takeoff point. Lower takeoff point tube folded over to block. Set up desired flow, wind up suction to bring upper pressure takeoff to zero. Then plug lower takeoff tube to Manometer other input. Take pressure reading.

Just did one to test. The 30 x 4mm Calibrator connected as above. With 20L going through it but suction off, the pressure at upper takeoff relative to air was 112mm. Balance that down to zero, connect up lower takeoff point and pressure across the calibrator is 60.

Ah but, pull off the vacuum pump and the pressure across the calibrator is still 60! And the flow still 20L. Doesn't this tell us that the back pressure across the Calibrator is not enough to bother the Flow Meter? So our "Calibrator open to air" tests can be relied upon?

And that we can safely retire the Sucker? It's demonstrated that it isn't necessary?

Ah, but all very well, I hear you mutter. I asked for a reading of the Calibrator upstream from the Flow Gauge. Oh alright, I respond petulantly, but dutifully reconfigure in that mode. Ooooh, interesting. I'm now seeing about 50mm of pressure across the Calibrator at 20L. So, two possibilities (at least!):
1) the relatively small resistance or back pressure of the Calibrator is messing with the Flow or the Flow Meter.
2) the relatively high back pressure of the Flow Meter is messing with the Calibrator or perhaps the Manometer (though it didn't complain).
3) unknown, suggestions welcomed.

With 20L of flow still running through the Calibrator and Flow Meter, I pull out the calibrator and replace it with a coupler (about 8mm diam bore, 45mm long). Flow Gauge still reads 20L, pressure meter now reads 2mm. I reckon it's Option No 2) above.

Heh heh, this is so much like any other significant science I've been involved in. 99% of the effort goes into developing the hardware and procedures. The actual experiment is over in minutes.

[trill]

. . . The digital flow meter you were looking at is based (if I understand correctly) on cooling effect of air flow over a heated sensor.

Yes, I've looked at 2 manufacturers, they describe how the MEMS sensors work. Forced convection.

What happens to that cooling effect if we increase the pressure and the density of the air flowing through it? Do they talk about that in the specs?

Indirectly. The spec sheet claims a "pressure loss" of less than 1 kpa (that's 1/100th of 1 atmosphere).

So, with such a low-pressure-loss sensor, the change in pressure and density will be very tiny. No need to worry about huge pressure+volume+density changes.

I imagine that at least its resistance to air flow is probably pretty low. Again, do they talk about that in the specs?

See above.

I have been wondering if we should concentrate on measuring pressure rather than flow for our tests. I know Tunborough prefers flow, but if we could use calibrators to predict the relationship, the Manometer is non-intrusive and easy to use. But lets see where the journey takes us....

My understanding of the model-in-progress contributed by Tunborough is that each whistle has a "coefficient" (K, Cd, WR - cousins all). Honestly, that was my hunch at the start of this thread. However, thinking more, I came to believe a flow-dependance (descending Cd) would be reasonable. Big factor ? I don't know. Depends on how the flow model will be used. But, we observed direct measurements supporting that once the "shifted" tape was fixed.

My perspective is: We are so close . . .

We've got: reliable pressure measurements, a well-exercised test+data-collection protocol, willing analysts (on 3 continents !), and a model-in-progress. With a good flow sensor, we could have a very clear picture.

I mean, taking a step back, the existing flowmeters are kind of a blurry lens, but they have allowed plenty of progress. And, frankly, it's been fun trying to puzzle-out all the effects. Maybe they're "good enough" for hobby-hackers.

But, really, wouldn't it be nice ?

ps: the Amazon seller allows refunds for 30 days . . . my offer to cost-share still stands.

[Terry McGee]

What happens to that cooling effect if we increase the pressure and the density of the air flowing through it? Do they talk about that in the specs?

Indirectly. The spec sheet claims a "pressure loss" of less than 1 kpa (that's 1/100th of 1 atmosphere).

So, with such a low-pressure-loss sensor, the change in pressure and density will be very tiny. No need to worry about huge pressure+volume+density changes.

OK, so 102mm compared to something far exceeding 1400mm in the current Flow Gauges. And that presumably at 100L/Min, so we'd be seeing considerably less again. So that's unlikely to bother anything upstream from it.

But I was thinking supposing you had a high resistance Calibrator or whistle at the end of the chain (ie after the Flow Gauge), is it likely that it's back pressure would bother this kind of flow gauge. I can imagine that a sensor based on heat flow might be very sensitive to operating pressure and therefore air density. They don't seem to give a "sensitivity to operating pressure" factor in the specs.

I was amused by the "Turn Down Factor" given as 80:1. My mind immediately interprets that as the number of girls you have to ask before you get a dance at the Prom. But it seems it means the gauge can be used down to Full Scale/80, about 1.25L/Min. That's very acceptable. And it's resolution looks very good too.

I have been wondering if we should concentrate on measuring pressure rather than flow for our tests. I know Tunborough prefers flow, but if we could use calibrators to predict the relationship, the Manometer is non-intrusive and easy to use. But lets see where the journey takes us....

My understanding of the model-in-progress contributed by Tunborough is that each whistle has a "coefficient" (K, Cd, WR - cousins all). Honestly, that was my hunch at the start of this thread. However, thinking more, I came to believe a flow-dependance (descending Cd) would be reasonable. Big factor ? I don't know. Depends on how the flow model will be used. But, we observed direct measurements supporting that once the "shifted" tape was fixed.

My perspective is: We are so close . . .

We've got: reliable pressure measurements, a well-exercised test+data-collection protocol, willing analysts (on 3 continents !), and a model-in-progress. With a good flow sensor, we could have a very clear picture.

I mean, taking a step back, the existing flowmeters are kind of a blurry lens, but they have allowed plenty of progress. And, frankly, it's been fun trying to puzzle-out all the effects. Maybe they're "good enough" for hobby-hackers.

But, really, wouldn't it be nice ?

ps: the Amazon seller allows refunds for 30 days . . . my offer to cost-share still stands.

Let's see what the others make of today's tests. Are we near enough, or do we need sharper tools?

[trill]

. . . But I was thinking supposing you had a high resistance Calibrator or whistle at the end of the chain (ie after the Flow Gauge) . . .

Why do we need a high-resistance calibrator ?

Why is a "high resistance" calibrator needed for operating condx far-removed from the pressure+flow of a whistle-player ?

I thought the calibrators were needed as a check on the flowmeters.

The spec sheets allows for operating pressures up to .6mPa (~6 atmospheres).

It's true, the spec sheet states the accuracy given is for at 20C + 1atm. But a whistler blows barely above 1 atm.

My belief is that better knowledge of the flow will allow a better model.

[david_h]

Thoughts from the GMT timezone. I once worked briefly on a software project where the coders where in the UK and testers in Hong Kong. Never a break. A reminder that the whole world never sleeps.

Operating pressure clearly effects the flow meter reading in a way we don't understand. The pressures that matter will be the ones either side of the float. So it's the downstream-side pressure that matters most. My guess is that the backpressure is coming mainly from the adjuster on the inlet even when fully open. The scale spacings are interesting and tell us something non linear (even if only the bore taper) is going on. I would tend to accept that the makers know what they are doing - even if they are just calibrating what, on average, comes out of the mould.

If the flowmeter is downstream of the calibrator/whistle and open to atmosphere there should be a consistent (but not necessarily linear) relationship between actual flow and the scale reading for each test. Measuring the pressure across a calibrator is no problem - but unless the water manometer comes off the shelf we won't know what the pressure is relative to atmosphere so we don't know rho, the density of air. It's also complicated to set up with a whistle (I am wondering if some container from the kitchenware section of the supermarket could be modified). There might be some latency if the container volume was large but that should be no problem for a blowing machine.

If the flowmeter is upstream of the calibrator/whistle the setup is simpler, especially for a whistle. We then have a flow reading that is a function of both actual flow and pressure. However, we are measuring the outlet side pressure on the flow meter and do know the density of air in the calibrator. It is also not much problem to leave the flow meter in the system and record it even if only planning to use the pressure - we might find we could use it if more information becomes available.

So I think have the flowmeter above the calibrator (and could probably use its adjuster to regulate flow and pressure). A full set of reading with the calibrators (including a zero length one) could then be enough to distinguish between flow measurement irregularities and flow dynamics changes in the calibrators. It would be possible to put a resistance downstream of the calibrator (with manomter in differential mode across the calibrator) to see how operating at an (unknown) higher pressure moved any features of interst on the graphs. Or to bring out the water manometer and know the pressure. It also saves Terry having to make and leak test constructions involving Tupperware boxes or whatever.

My main interest in the modelling is concern that we may be modelling noise or alternatively throwing out the baby with the bath water. I am also trying to get my head round the maths in a way that doesn't obscure text book first principles and intuition**. For example the statement of Bernoulli's equation I gave can cope with input velocity without involving K. It also highlights an approximation that we are making; we are assuminng flow of an incompressible fluid so rho is the same on both sides of the equation/calibrator. The shenanigins with the flow meters have reminded us that it is not.

**I suspect one of the reasons for mathematical rearrangements (complicated I suspect) that allow pressure losses to be accumulated into K is the earlier need to use a slide rule (younger readers may need to look that one up as well) and maybe ready-reckoner tables (ditto). Using a spreadsheet I tend to keep things separate.

[Terry McGee]

. . . But I was thinking supposing you had a high resistance Calibrator or whistle at the end of the chain (ie after the Flow Gauge) . . .

Why do we need a high-resistance calibrator ?

Why is a "high resistance" calibrator needed for operating condx far-removed from the pressure+flow of a whistle-player ?

I thought the calibrators were needed as a check on the flowmeters.

And on the validity of the system at large. It would make sense to me to have a slightly higher resistance Calibrator than the hardest blowing whistle, a slightly lower resistance one than the easiest blowing whistle, and one in the mid range. And I'm still planning to do an orifice plate, just in case we learn something!

The spec sheets allows for operating pressures up to .6mPa (~6 atmospheres).

So it would definitely be safe in our system. But how accurate?

It's true, the spec sheet states the accuracy given is for at 20C + 1atm. But a whistler blows barely above 1 atm.

The bit that bothers me is that the whistler covers a range of pressures, and if that were to be enough to introduce an error, it will be an error that varies with the pressure, not just a simple offset.

My belief is that better knowledge of the flow will allow a better model.

Agree, but we'd have to somehow convince ourselves that it can be trusted at the slightly higher pressures than a whistle or calibrator might reflect back.

You'd think that all flow meters would be capable of transcending pressure-related errors wouldn't you. How do you make air flow without pressure?

[david_h]

Are all requirements the same? I think Tunborough, who wants to be able to produce a whistle geometry from theory and maybe some empirical factors that might apply to all whistles, has the most stringent requirement.

If one has a whistle, to such design or otherwise, and have measured its characteristics there is a lesser requirement on the model - to predict what geometrical adjustments are needed to make a specific change to its characteristics without changing things we are happy with.

An even less stringent requirement is to be able to describe whistles - pressure, air requirements, tuning, tone, volume etc in a more quantitative way. So we have a better idea how another model of whistle will compare with what we have.

[trill]

. . . The bit that bothers me is that the whistler covers a range of pressures, and if that were to be enough to introduce an error, it will be an error that varies with the pressure, not just a simple offset. . .

Well, our old Gen takes ~200 mmH2o do get D7. That's 200 out of ~10,000 mm for 1atm. That's like 2%.

You're worried ? Really ?

. . . but we'd have to somehow convince ourselves that it can be trusted at the slightly higher pressures than a whistle or calibrator might reflect back. . .

Well, the mfr claims "(2+.5FS)%".

I have a query into a sales rep.

[david_h]

Theoretical question. A quote from the University of Wollongong paper linked on an adjacent thread.

" To further improve the design, the wind-way or narrow duct through which the breath passes, was increased to allow a higher volumetric airflow rate to account for any wall smoothness issues and to aid in the cleaning process."

Do we then have to blow more air - but at the same pressure so as to maintain the required velocity - or does the increased volumetric flow in the jet somehow compensate? I have been looking online in the last couple of days but didn't find anything.

[Tunborough]

Theoretical question. A quote from the University of Wollongong paper linked on an adjacent thread.

" To further improve the design, the wind-way or narrow duct through which the breath passes, was increased to allow a higher volumetric airflow rate to account for any wall smoothness issues and to aid in the cleaning process."

Do we then have to blow more air - but at the same pressure so as to maintain the required velocity - or does the increased volumetric flow in the jet somehow compensate? I have been looking online in the last couple of days but didn't find anything.

Yes, you have to maintain the same velocity, so a bigger windway means bigger air flow. Air pressure would be the same or maybe a bit less since there would be a little less loss through the windway.

[david_h]

Thanks. If windway exit to labium distance is changed does that then change the velocity required for the same note?
I am wondering, in the context of 'resistance', if an 'easy blowing' whistle is one that gets through the air faster even if it has the same 'backpressure' or is one that requires less pressure.